1. Field
The invention relates to methods and apparatus for producing three-dimensional (3-D) images (volume displays) from a set of 2-D images, and more particularly to the use of off-axis diffractive optical elements (preferably multiple component holographic optical elements called mcHOEs herein) in special combination with an off-axis array of a plurality of illuminated (or radiative) components of objects to be synthesized into 3-D images of the objects themselves. The invention is useful in a wide variety of modes, including but not limited to, applications in scientific, technical, medical, entertainment, educational, commercial and advertising fields, and more particularly to volume microscopy, 3-D tomogram synthesis, 3-D games, and 3-D holographic television and movies, to name a few.
2. Description of the Prior Art
Apparatus for realistically presenting three-dimensional images of objects have been explored extensively since the beginnings of photography. Excellent reviews of the progress of this endeavor, ranging from stereophotography to holography, are available; for instance in Three Dimensional Imaging Techniques by T. Okoshi.
One particular trend in three-dimensional imagery concerns synthesizing a 3-D image of an object from a set of representations of components of that object; a typical, example of this technique is provided in U.S. Pat. No. 3,428,393, for instance. That patent describes the basic goals of this prior art and provides a description of the range of objects that can be synthesized and the kinds of components of an object that can be used in the synthesis. A brief review of the goals of the prior art and some definitions of terminology will be made here for the purpose of this exposition.
An "object" is in general any three-dimensional collection of points to which some value relating to intensity can be assigned; it may be described by a mathematical formula or in a computer as a set of points, or it may be a physical object such as a biological specimen. Components of that object are any subset of the points comprising the object, and a representation of the component is some physical embodiment capable of effecting a wave front corresponding to the points of the component. For instance, the object may be a biological specimen, the components may be serial sections made through the object with a microtome, and the representations of the components may be photographic transparencies of these sections. In another instance, the object may be a set of points in a computer corresponding to a computerized axial tomography generated portrait of the 3-D distribution of body density in a patient. The components may be the set of 2-D intensity pictures generated by mathematically intersecting a plane at various depths within the 3-D collection of points, and the representations of the components may be intensity modulated regions of phosphor on the face of a CRT, or a photographic transparency of such a CRT screen. The goals of the synthesis of an image of the object from the representations of the components is to provide an observer with an image of each of the components in such a way as to create an effect of a single 3-D distribution.
A wide range of physical embodiments of this idea have been proposed. The prior art techniques all require moving parts to effect the synthesis. In some methods, an image of the different planar sectional components of an object is focused onto a screen, and some part of the system moves to create, for successive sections representing different depths within a specimen, an image of the screen at different distances from the observer. When this process is performed at a frequency above flicker fusion, an image of all components is visible simultaneously at their appropriate relative depths; this set of images comprises the 3-D object image. In other systems, for example, those described in U.S. Pat. No. 3,493,290, an optical element is deformed, and this changes the focal length of the element: components viewed with a different focal length appear to be a different distance from the observer. When the process of presenting different object components to different focal length optical elements (i.e., the same element in different states for deformation) occurs above the flicker fusion rate, one sees all components simultaneously, at a distance corresponding to the position within the object which the component would occupy.
Aside from the moving parts, bulkiness and expense of the above embodiments, the problem exists of presenting the different component images to the system, and thereby to the observer. This problem has been approached by others in a number of ways. In one approach the representations of the object components may exist physically separately, and are presented to the input aperture of the viewing system by physical movement of the representations, as in a moving film loop; or the representations may remain motionless and be imaged by a moving optical element, such as a rotating prism; or they may reside on a CRT screen, in which case successive components are displayed at different times on the CRT. All of these prior art methods require complicated apparatus to assure a synchronization of the presentation of the components to the viewing apparatus at the precise moment when the image of that component will appear at the proper depth within the complete 3-D image.
In the deformable mirror varying focal length system, the components are successively displayed on a CRT. The electronics bandwidth required to refresh the CRT once for each object component while meeting the flicker fusion repetition requirements is a limiting factor in the achievable resolution of each component, and hence of the final object image. One cannot use a number of CRT-electronics assemblies "in parallel" to increase the effective bandwidth, because only one CRT can exist at the input aperture of the deformable optical element. At the present time, the bandwidth of the CRT circuitry is the limiting factor for final object image resolution.
The prior art does not provide any technique for creating an image of a 3-D object by operating on representations of object components without the use of moving parts and complex synchronization mechanisms; nor does the prior art describe any technique for synthesizing the image of a 3-D object by acting on all object components simultaneously. Other patents which do not either address the problems or provide for solutions of the type herein are U.S. Pat. Nos.: 3,743,376; 4,094,577; 4,190,856; 3,493,290; 4,130,832; 3,807,829; 4,054,356; and 3,462,213. Basic background in the holographic field may be found in "White Light Holograms," an article by Emmett N. Leith in Scientific American, Oct. 1976, pp. 80-88, 92-95; and Handbook of Optical Holography (Academic Press 1979), Ch. 10.8, "Optically Recorded Holographic Optical Elements" by Donald L. Close.